Preventing carbon ao migration by limiting surface porosity

ABSTRACT

In the manufacture of carbon-carbon composite brake discs, migration of anti-oxidant substances into the friction surfaces is prevented by limiting or eliminating surface porosity in the carbon-carbon composite brake materials. The method includes infusing a suitable resin into pores in surface layers of the carbon-carbon composite disc and then charring the resin-infused disc to convert the resin in the pores to pyrolytic carbon. The resin may be infused into the carbon disc by submerging the disc in a molten resin. Prior to submerging the disc in the molten resin, the disc may subjected to a vacuum to remove air from the pores. While the disc is submerged in the molten resin, the pressure in the pressurizable vessel may increased to force the molten resin into the open porosity of the disc.

FIELD OF THE INVENTION

Carbon-carbon composites are commonly used as brake friction materialsin the aerospace industry. Carbon-carbon composites provide a goodcombination of high strength, low weight, and effective frictionproperties. However, they are vulnerable to oxidation at elevatedtemperatures. This invention relates to the manufacture of carbon-carbonbrake discs. More particularly, this invention relates to brake discswhich are coated with antioxidant compositions during the course oftheir preparation for use in braking systems.

BACKGROUND OF THE INVENTION

In the product of carbon-carbon composite articles, a preform is madewith carbon fibers or carbon fiber precursors, in textile form or in theform of loose fibers, and the resulting fibrous matrix is densified,typically, by filling it with a resin which is then carbonized and/or byfilling it with chemical vapor. The densification process is generallyrepeated until the pores in the preform are so narrow that furtherpenetration by densifying agents is impractical.

Antioxidant treatments are required to protect non-friction surfaces ofcarbon-carbon composite brake friction materials, due to the highoperating temperatures of braking systems utilizing these materials.Oxidation may be minimized by a technological process of applyinganti-oxidation (“AO”) solution to the non-friction surfaces of the brakediscs. Unfortunately, the most common AO solutions have a tendency tomigrate through the porosity of the carbon-carbon friction material inthe presence of atmospheric humidity. This migration leads tocontamination by the AO solution of the friction surfaces of thecarbon-carbon composite brake materials, thus decreasing the frictionproperties of the brake.

Combinations of phosphoric acid and various metal phosphates arecommonly used for such antioxidant treatments. Unfortunately, these samematerials have adverse effect on braking effectiveness. Specifically,they lower friction coefficients of the carbon-carbon compositematerials to which they are applied. While this is not a problem on thenon-friction surfaces of the brake discs, it is very much a problem whenthe antioxidant material contacts the friction surfaces thereof. Becauseconventional antioxidant treatments are virtually invisible oncarbon-carbon composites in their cured state, accidental applicationthereof to the friction surface can go undetected, resulting in adverseperformance of the brake friction material.

Currently employed measures to prevent AO migration include (1) limitingthe amount of phosphoric acid in the AO solution and (s) using high AOchar temperatures. However, both of these methods limit theanti-oxidation effectiveness of the AO solution, and they createtechnological problems during the manufacturing process.

U.S. Pat. No. 7,160,618 relates to an AO system which is resistant tohumid migration. In the present invention, in contrast, migration isavoided no matter what AO system is used. U.S. Pat. No. 7,118,805 and US2007/0218208 similarly relate to formulating the AO system in such a waythat migration would not occur. The present invention allows the use ofany AO system. US 2007/0199626 discusses, in paragraphs [0030] to[0032], the established art of creating a carbon-carbon compositematerial. The use of various resins to densify a precursor matrix iswell known in industry, but is not widely practiced because theresultant compose density tends to be low. The low density occursbecause carbonizing the resins closes the porosity in the compositebody, thereby inhibiting subsequent further densification of thecomposite body.

SUMMARY OF THE INVENTION

The present invention prevents AO migration by limiting or eliminatingsurface porosity in the carbon-carbon composite brake materials, therebyremoving the possible migration paths.

The present invention provides an improvement in the manufacture ofcarbon-carbon composite brake discs. The method of this inventionincludes the steps of infusing phenolic resin or epoxy resin or cyanateester resin into pores in surface layers of a carbon-carbon compositefriction disc; and subsequently charring the resin-infused disc toconvert the resin in the pores to pyrolytic carbon. The charring stepmay be carried out at any convenient temperature, typically at atemperature in the range of 500° C. to 900° C.

In the method of the present invention, the resin may be infused intothe carbon-carbon composite friction disc by submerging the disc in amolten resin in a pressurizable vessel so that the molten resin can flowinto the open porosity of the disc. Prior to submerging the disc in themolten resin, the disc may subjected to a vacuum, for instance, a vacuumof 10 torr, to remove air from the inner porosity of the disc. While thedisc is submerged in the molten resin, the pressure in the pressurizablevessel may increased, for instance, up to 3000 psi, typically, to 30-300psi, in order to force the molten resin into the open porosity of thedisc. Vacuum or pressure may be used alone, or both may be used in orderto achieve pore blocking in the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be more filly understood from the detaileddescription given below and the drawings that accompany thisspecification. The drawings are given by way of illustration only, andthus are not limiting of the present invention. The drawings are notnecessarily to scale.

FIG. 1A is a schematic representation of a carbon-carbon compositeshowing pores open to the atmosphere.

FIG. 1B is a schematic representation of the carbon-carbon composite ofFIG. 1A, showing its pores filled with resin.

FIG. 1C is a schematic representation of the carbon-carbon composite ofFIG. 1B in which the resin in its resin-filled pores has beencarbonized, providing a carbon-carbon composite wherein pores formerlyopen to the atmosphere no longer exist.

DETAILED DESCRIPTION OF THE INVENTION

A preferred method of implementing the present invention is to infusephenolic resin or epoxy resin or cyanate ester resin into the surfacelayers of carbon-carbon composite friction discs, followed by a charheat treatment to convent the resin to pyrolytic carbon. One advantageof cyanate ester resins is that they cure without the development ofgas. One method for infusing the resin into the carbon-carbon compositefriction discs is to submerge the disc in a molten resin and thenincrease the pressure in the pressurizable vessel. This forces themolten resin into the open porosity of the disc. Subsequent charcarbonizes the resin in place in the pores. Pyrolytic carbon has verylow porosity. Accordingly, the pyrolytic carbon effectively closespossible migration routes for the AO solution.

In accordance with the present invention, at some stage of themanufacturing process, the brake disc is treated with an antioxidantcomposition. The “working surface” of the brake disc should present themaximum possible frictional properties to the brake pad. The terminology“working surface of the brake disc” refers to that portion of the brakedisc that frictionally engages with a brake pad during a brakingoperation. Accordingly, the present invention contemplates processingthe brake disc, before incorporating it into a brake system, to avoid orinhibit the antioxidant composition from reaching the working surface ofthe disc.

The production of carbon-carbon composite materials, including brakefriction materials, has been described extensively in the prior art. Onecommonly used production method comprises molding a carbon fibercomposite with a carbonizable resin, e.g. a phenolic resin, carbonizingthe composite “preform”, and then densifying the resulting porousmaterial using chemical vapor infiltration (CVI) and/or resinimpregnation processes. Another method comprises building up a fiberpreform with textile materials and subsequently densifying the preformusing a CVI process. Different structural types of carbon (graphitic,glassy, and pyrolytic) comprise the brake disc, which is somewhatporous. Further densification can be accomplished with, e.g., furfurylalcohol infiltration or through incorporation into the carbon matrix ofceramic additives via infiltration with colloidal ceramics and theirsubsequent conversion to refractory materials.

Carbon-carbon brake disc friction performance is dictated by the carbonmicrostructure which arises from the manner in which the brake disc ismanufactured. The amount of graphitization, for instance, candramatically affect frictional and wear properties. Overall brakeperformance is particularly affected by the individual components,including fibers and types of matrix materials, at the friction surface.

One source of problems with these carbon composites is that they havelow resistance to oxidation, by atmospheric oxygen, at elevatedtemperatures, that is, temperatures of 500° C. (932° F.) or higher.Oxidation not only attacks the surface of the carbon-carbon compositesbut also enters pores that invariably are present in such structures andoxidizes the carbon fibers adjacent to the pores and surfaces of thepores, thereby weakening the composites.

Exterior surfaces of carbon-carbon composites are therefore sometimescoated with a ceramic material such as silicon carbide to prevent entryof oxidizing agents such, as molecular or ionic oxygen from theatmosphere, into the carbon-carbon composites. Silicon carbide and otherantioxidant coatings are described in detail in U.S. Pat. No. 4,837,073.The exterior surfaces of carbon-carbon composites may be, alternatively,coated with a glass-forming seal coat such as a boron or boron/zirconiumsubstance. Borate glasses have also been used from the protection ofcarbon-carbon composites against oxidation. U.S. Pat. No. 5,208,099describes antioxidant coatings that are formed from a SiO₂—B₂O₃ geland/or sol having a SiO₂:B₂O₃ molar composition of 60-85:40-15. Borateglass antioxidant compositions are moisture-resistant andoxidation-resistant coatings composed of 40-80 weight-% B₂O₃, 5-30weight-% SiO₂, 7-20 weight-% Li₂O, and 7-10 weight-% ZrO₂ are describedin detail in U.S. Pat. No. 5,298,311.

U.S. Pat. No. 6,737,120 (Golecki) relates to carbon fiber or C—Ccomposites that are useful in a variety of applications. Golecki teachesmethods of protecting such composites against oxidation by coating themwith fluidized-glass type mixtures. The fluidized-glass mixtures aremaintained as liquid precursors and are applied to components formed ofcarbon fiber or C—C composites. Once coated with the precursors, thecoated C—C components are heat-treated or annealed for one or morecycles through a series of gradual heating and cooling steps. Thiscreates glass coatings having thicknesses of about 1-10 mils. Thethicknesses of the glass coatings may be varied by varying thecomposition of the fluidized glass precursor mixtures, the number ofapplication cycles, and/or the annealing parameters.

The Golecki patent teaches that the fluidized glass materials maycomprise such materials as borate glasses (boron oxides), phosphateglasses (phosphorus oxides), silicate glasses (silicon oxides), andplumbate glasses (lead oxides). These glasses may include phosphates ofmanganese, nickel, vanadium, aluminum, and zinc, and/or alkaline andalkaline earth metals such as lithium, sodium, potassium, rubidium,magnesium, and calcium and their oxides, and elemental boron and/orboron compounds such as BN, B₄C, B₂O₃, and H₃BO₃. By way of example,Golecki discloses a boron-containing liquid fluidized glass precursormixture that includes 29 weight-% phosphoric acid, 2 weight-% manganesephosphate, 3 weight-% potassium hydroxide, 1 weight-% boron nitride, 10weight-% boron, and 55 weight-% water.

U.S. Pat. No. 6,455,159 (Walker and Booker) likewise relates toantioxidant systems for use with carbon-carbon composites and graphiticmaterials. The Walker and Booker patent has among its objectives theprotection of antioxidant-coated carbon-carbon composites or graphitesat elevated temperatures up to and exceeding 850° C. (1562° F.), as wellas the reduction of catalytic oxidation at normal operatingtemperatures. Walker and Booker achieve these objectives by employing apenetrant salt solution which contains ions formed from 10-80 wt % H₂O,20-70 wt % H₃PO₄, 0.1-25 wt % alkali metal mono-, di-, or tri-basicphosphate, and up to 2 wt % B₂O₃. Their penetrant salt solutions alsoinclude at least one of MnHPO₄.1.6H₂O, Al(H₂PO₄)₃, and Zn₃(PO₄)₂, inweight-percentages up to 25 wt %, 30 wt %, and 10 wt %, respectively.

The entire contents of U.S. Pat. No. 4,837,073, U.S. Pat. No. 5,208,099,U.S. Pat. No. 5,298,311, U.S. Pat. No. 6,737,120, and U.S. Pat. No.6,455,159 are hereby expressly incorporated by reference.

Carbon-carbon composites are generally prepared from carbon preforms.Carbon preforms are made of carbon fibers, formed for instance ofpre-oxidized polyacrylonitrile (PAN) resins. These fibers can be layeredtogether to form shapes, such as friction brake discs, which shapes arethen heated and infiltrated with methane or another pyrolyzable carbonsource to form the C—C composite preforms. Carbon-carbon compositesuseful in accordance with the present invention typically have densitiesin the range of from about 1.6 g/cm³ through 1.9 g/cm³. Methods ofmanufacturing C—C composites are generally well known to those skilledin the art. A good reference in this area is: Buckley et al.,Carbon-Carbon Materials and Composites, Noyes Publications, 1993. Theentire contents of this publication are hereby expressly incorporated byreference.

For purposes of illustration only, the C—C composite brake disc preformmay be fabricated from woven fabric panes of pitch-based Amoco P30Xcarbon fiber tows in a harness satin weave or from a pitch-based NipponXNC25 in a plain weave. The tows are rigidized with a few weight-%carbon-containing resin, such as epoxy Novolac. The material is thencarbonized at a temperature in the range of 800-1000° C. and densifiedby carbon CVD. The resulting materials is then annealed in an inert gasat a temperature in the range of 1600-2600° C. This process creates aC—C composite component that is adaptable for use in high temperatureenvironments when it is properly protected against oxidation. It isunderstood that the oxidation protective coating system of the presentinvention is applicable to C—C composite components regardless of howthe C—C composite components are fabricated.

The Resins

A variety of commercially available resins may be used to practice thepresent invention.

For instance, the resin may a phenolic resin, for instance, a two stagedry phenolic resin, such as a Novolac resin, having a molecular weightof about 3500 to 4000 or having a molecular weight of about 7500 to9000, or a one stage liquid phenolic resin, such as a Resol resin,having a molecular weight of about 200 to 500.

The resin may be an epoxy resin, for instance, an epoxy resin having amolecular weight of about 450 to 4000.

The resin may be a cyanate ester resin, for instance, formed bypolymerizing a compound of the formulaNCOC₆H₅—CH₂—(NCOC₆H₅—CH₂)_(n)NCOC₆H₅ where n is a number from 0 to 20.The bonds to the methylene groups which link the benzene rings (C₆H₅)can in principle go out to the cyanate groups from the ortho, meta, orpara positions. Linking typically takes place through the ortho and parapositions. The compounds are typically in the form both of oligomermixtures (with different values of n) and of isomer mixtures (withdifferent linkage patterns, preferably o- or p- for the terminal benzenerings and o-,o- or o-,p- for the nonterminal benzene rings). Suchcyanate esters are available commercially, for example, under thedesignation PRIMASET® from the company Lonza AG, Basle, Switzerland.

Persons skilled in the art will readily recognize resins such as theabove that can be used to implement the present invention. The primaryqualifications of the resin to be used are that, when molten, is musthave a viscosity such that it can be into the open porosity of thesurface layers of the disc, and it must be convertible into pyrolyticcarbon by charring.

The Drawings

The accompanying drawings may be used to facilitate understanding of thepresent invention. FIG. 1A is a schematic representation of acarbon-carbon composite showing pores open to the atmosphere. FIG. 1B isa schematic representation of the carbon-carbon composite of FIG. 1A,showing its pores filled with resin. FIG. 1C is a schematicrepresentation of the carbon-carbon composite of FIG. 1B in which theresin in its resin-filled pores has been carbonized, providing acarbon-carbon composite wherein pores formerly open to the atmosphere nolonger exist. It should be understood that the drawings are not toscale, particularly with respect to the relative size of the pores, andthat internal pores in the carbon-carbon composite are not depicted.However, the drawings portray the intent and mechanics of the invention.The drawings make the point that once the pores in the composite whichare open to the air are sealed, there is no capillary path for theliquefied antioxidant composition to migrate into the composite.

EXAMPLES Example 1

A carbon-carbon composite brake disc preform having an external diameterof 20 inches, an internal diameter of 14 inches, a thickness of 1.5inches, and a density of 1.7 g/cc is provided. The preform is made fromnonwoven PAN fabric. The disc is submerged in a molten phenolic resin ina pressurizable vessel and then pressure in the pressurizable vessel isincreased to 200 psi in order to force the molten resin into the openporosity of the surface layers of the disc. Subsequently, theresin-infused disc is charred at 800° C. to convert the resin within thepores of the disc to pyrolytic carbon.

Example 2

A carbon-carbon composite brake disc preform having an external diameterof 18 inches, an internal diameter of 10 inches, a thickness of 1.25inches, and a density of 1.75 g/cc is provided. The preform is made fromchopped pitch fibers. The disc is submerged in a molten epoxy resin in apressurizable vessel and then pressure in the pressurizable vessel isincreased to 50 psi in order to force the molten resin into the openporosity of the surface layers of the disc. Subsequently, theresin-infused disc is charred at 550° C. to convert the resin topyrolytic carbon.

Example 3

A carbon-carbon composite brake disc preform having an external diameterof 21 inches, an internal diameter of 15 inches, a thickness of 1.75inches, and a density of 1.5 g/cc is provided. The preform is made fromnonwoven PAN fabric. The disc is submerged in a molten mixture ofphenolic and epoxy resins in a pressurizable vessel and then pressure inthe pressurizable vessel is increased to 150 psi to force the moltenresin into the open porosity of the surface layers of the disc.Subsequently, the resin-infused disc is charred at 750° C. to convertthe resin to pyrolytic carbon. cl Example 4

A carbon-carbon composite brake disc preform having an external diameterof 22 inches, an internal diameter of 12 inches, a thickness of 1.75inches, and a density of 1.7 g/cc is provided. The preform is made fromnonwoven PAN fabric. The disc is placed in a pressurizable vessel, andthe vessel is evacuated to 10 torr and held at that vacuum to evacuateair from the internal pores within the disc. The vacuum is thenreleased, and molten phenolic resin is added to the pressurizablevessel. The pressure in the pressurizable vessel is then increased to300 psi in order to force the molten resin into the open porosity of thesurface layers of the disc. Subsequently, the resin-infused disc ischarred at 900° C. to convert the resin to pyrolytic carbon.

Example 5

A carbon-carbon composite brake disc preform having an external diameterof 18 inches, an internal diameter of 10 inches, a thickness of 1.25inches, and a density of 1.75 g/cc is provided. The preform is made fromchopped pitch fibers. The disc is placed in a pressurizable vessel, andthe vessel is evacuated to 10 torr and held at that vacuum to evacuateair from the internal pores within the disc. The vacuum is thenreleased, and molten cyanate ester resin is added to the pressurizablevessel. The pressure in the pressurizable vessel is then increased to100 psi in order to force the molten resin into the open porosity of thesurface layers of the disc. Subsequently, the resin-infused disc ischarred at 650° C. to convert the resin within the pores of the disc topyrolytic carbon.

Each of the carbon-carbon composite brake discs produced as describedabove is immersed in a conventional anti-oxidation solution, care beingtaken to ensure that only the body and sides of the disc are submerged,and that the working surface of the disc is not contacted by theanti-oxidation solution. The anti-oxidation solution-coated discs arethen dried in an oven at 100° C. in order to set the anti-oxidant intothe non-working surfaces of the discs. Inspection of the workingsurfaces (friction surfaces) of the discs confirms that no anti-oxidanthas reached the friction surfaces.

8 Persons skilled in the art will readily recognize that additionalvariations of the above-described implementations may be reached withoutdeparting from the spirit and scope of the present invention.

1. A method of manufacturing a carbon-carbon composite brake disc whichcomprises the steps of: infusing phenolic resin or epoxy resin orcyanate ester resin into pores in surface layers of a carbon-carboncomposite friction disc; and subsequently charring the resin-infuseddisc to convert the resin in the pores to pyrolytic carbon.
 2. Themethod of claim 1, wherein the resin-infused disc is charred at atemperature in the range of 500° C. to 900° C.
 3. The method of claim 1,wherein the resin is a phenolic resin.
 4. The method of claim 1, whereinthe resin is an epoxy resin.
 5. The method of claim 1, wherein the resinis a cyanate ester resin.
 6. The method of claim 1, wherein the resin isinfused into the carbon-carbon composite friction disc by submerging thedisc in a molten resin so that the molten resin can flow into the openporosity of the disc.
 7. The method of claim 6 wherein, prior tosubmerging the disc in the molten resin, the disc is subjected to avacuum to remove air from the inner porosity of the disc.
 8. The methodof claim 7, wherein the disc is subjected to a vacuum of 10 torr priorto its submergence in the molten resin.
 9. The method of claim 1wherein, after the carbon-carbon composite friction disc is submergingin a molten resin in a pressurizable vessel, the pressure in thepressurizable vessel is increased to force the molten resin into theopen porosity of the disc.
 10. The method of claim 9, wherein thepressure is increased to 3000 psi to force the molten resin into theopen inner porosity of the disc.
 11. The method of claim 10, wherein thepressure is increased to at least to 30 psi and to as high as 300 psi toforce the molten resin into the disc.
 12. The method of claim 1, whereinthe carbon-carbon composite friction disc is subjected to vacuum toremove air from the inner porosity of the disc and subsequently theresin is infused into the disc by submerging the disc in a molten resinand then increasing the pressure in the pressurizable vessel to forcethe molten resin into the open porosity of the disc.